Buck Converter Cascode Stage Using Mixed MOSFET Breakdown Voltages
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Solution Overview
Problem
Typical buck regulator power converters require MOSFETs with high breakdown voltage, leading to increased RDSon and gate charge, which results in higher losses and performance limitations due to the need for thicker gate oxide and larger gate area in higher voltage MOSFETs.
Innovation Solution
A buck voltage converter design utilizing a cascode output stage with serially connected MOSFETs of different breakdown voltages, where a low-voltage switch is used in a high-voltage application, and core and I/O transistors with varying gate oxide thickness and breakdown voltages are employed to reduce blocking requirements and minimize gate charge impact.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If MOSFETs with high breakdown voltage are used to block full input voltage, then the blocking capability is improved, but RDSon and gate charge increase dramatically leading to higher losses
Solution Approach 1:
The patent divides the voltage blocking function into multiple segments by using a cascode configuration with two MOSFETs in series. The first MOSFET blocks a portion of the input voltage while the second MOSFET blocks the remaining voltage, allowing each device to operate at lower voltage stress and achieve lower RDSon values than a single high-voltage MOSFET would provide.
Solution Approach 2:
The patent applies different MOSFET types with optimized characteristics to different positions in the circuit. The first MOSFET uses a type optimized for low RDSon to minimize conduction losses, while the second MOSFET uses a type optimized for voltage blocking capability. This local optimization allows each component to perform its specific function with maximum efficiency.
2Reliability
If MOSFETs with high breakdown voltage are used, then the voltage blocking capability is improved, but gate charge increases due to thicker gate oxide and larger gate area
Solution Approach 1:
The patent segments the voltage blocking requirement between two MOSFETs, allowing each device to have thinner gate oxide and smaller gate area than a single high-voltage MOSFET would require. This segmentation dramatically reduces the total gate charge while maintaining the required voltage blocking capability.
Solution Approach 2:
The patent uses different MOSFET types with gate structures optimized for their specific roles. The first MOSFET uses a gate structure optimized for fast switching with lower gate charge, while the second MOSFET uses a gate structure optimized for voltage blocking. This local optimization minimizes total gate charge while maintaining reliability.
3Reliability
If higher voltage MOSFETs are used, then the breakdown voltage capability is improved, but RDSon per unit area increases proportionally to the square of breakdown voltage
Solution Approach 1:
The patent segments the high voltage blocking requirement into two separate MOSFETs, each handling a portion of the total voltage. This allows each MOSFET to operate at lower voltage stress where RDSon per unit area is much lower, significantly reducing total conduction losses compared to using a single high-voltage MOSFET.
Solution Approach 2:
The patent optimizes each MOSFET's channel characteristics for its specific voltage handling role. The first MOSFET uses channel dimensions and doping optimized for low RDSon to minimize conduction losses during the on-state, while the second MOSFET uses characteristics optimized for voltage blocking. This local optimization minimizes overall power dissipation.
Data Source
AI summary
A buck voltage converter is disclosed. The buck voltage generator includes a controller configured to generate one or more pulse width modulation (PWM) signals, and a plurality of serially connected switches configured to receive the PWM signals and to generate an output voltage signal at an output terminal based on the received PWM signals. The output voltage signal has an average voltage corresponding with a duty cycle of the PWM signals, a first switch of the plurality of serially connected switches has a first breakdown voltage and a second switch of the plurality of serially connected switches has a second breakdown voltage, and the first breakdown voltage is less than the second breakdown voltage.